There's a few routes.First, you can ask the producers:
Survivor Entertainment Group
9899 Santa Monica Blvd., PMB 2002
Beverly Hills, CA 91522
Couldn't hurt. Much. But it may be the most reliable route.
Second, you can try to buy them. StubHub has sold finale tickets before, although there's none up there now -- just looked -- and the same applies to eBay.
Third, find a former contestant. They occasionally have the connections to get a few, especially if they live in the area the finale's being shot in -- and if they can't personally spare one, they might know someone who could.
And lastly, be related to someone on the current edition. Still possible to execute if you're single, but you might want to hurry...